FE: Allow users to create a Premium trial - GitLab.com Billing Page Update

This is the frontend issue for #2728 (moved)

Success Criteria

1. Create Start free trial buttons on the GitLab.com Billing Page in the plan cards per the designs.
   1. The buttons should be connected to the proper trial plan (Premium vs. Ultimate) - BE issue: #2728 (moved)
2. The main Start your free trial blue button at the top of the Billing Page should be connected to the legacy Ultimate trial plan